The present invention relates generally to explosion-proof electrical fixtures having a circuit breaker switch for energizing and deenergizing the fixture, and more specifically to an interlock device used in such a fixture in order to ensure that a plug may be inserted or removed from the fixture only when the fixture is deenergized.
Explosion-proof fixtures are used in industrial environments containing hazardous substances like dust particles and gases. An electrical device such as a power source receptacle or switch is an arcing device, and sparks produced thereby may ignite the particles or gases to cause an explosion. Therefore, such devices are commonly housed in an electrical fixture consisting of a container and cover made from heavy-gauge metal, the two halves being securely connected so that any flame arising from an explosion inside the fixture cannot propagate outside the fixture to ignite the external environment.
Because a power source contained in the fixture delivers large voltage currents, it is often advisable for purposes of safety to include a circuit breaker switch so that the power source receptacle may be deenergized before a plug is inserted or removed. However, it is foreseeable that the operator of an electrical machine might forget to turn off the circuit breaker switch before inserting or removing a plug from the fixture receptacle, thereby running the risk of experiencing an electrical shock. As a reminder, it is possible to include in the fixture design an independent, hand-operated safety shaft, which engages the plug casing, and therefore must be disengaged before the plug casing may be inserted into or removed from the socket.
It is still conceivable, however, that the operator might disengage the safety shaft and insert or remove the plug without turning off the circuit breaker switch. Therefore, the safety shaft and circuit breaker may be interconnected by suitable means so that the safety shaft cannot be disengaged before the circuit breaker is turned off. Such a device is disclosed in U.S. Pat. No. 3,735,078 issued to Appleton et al., and assigned in common to the assignee of the present invention. Basically, the safety shaft and a rod operated by the circuit breaker handle feature undercut regions in the respective shafts, creating cooperating abutment surfaces so that the safety shaft can be moved only if the circuit breaker shaft has been moved to a predetermined "off" position.
However, the undercut designs of the safety shaft and circuit breaker shaft do not prevent the safety shaft from being placed in a position without a plug inserted in the receptacle housing, such that the circuit breaker can be turned on. Therefore, a second reciprocating shaft is included in the U.S. Pat. No. 3,735,078 design, which is spring biased into interlocked engagement with the safety shaft such that it normally secures the safety shaft in a disengaged position whereby the circuit breaker is locked in the "off" position. But when a plug is inserted in the receptacle, it moves the second reciprocating shaft to a position such that an undercut therein allows the safety shaft to drop to an engaged position with the plug casing, thereby permitting the circuit breaker to be turned on. In order to remove the plug from the receptacle, the circuit breaker must be turned off and the safety shaft lifted, whereupon the spring biased second reciprocating shaft engages the safety shaft in the disengaged position once the plug is removed.
This prior art design, though, requires a second set of undercuts to be machined into the safety shaft and second reciprocating shaft. Moreover, a separate actuating means is needed to move the second reciprocating shaft, which in the U.S. Pat. No. 3,735,078 takes the form of an independent lever rod, which does not always work smoothly. Furthermore, the undercut regions of the safety shaft, circuit breaker shaft, and second reciprocating rod must be machined so that the three shafts can meet in precise, cooperative alignment, or else the device will not work properly.